OBJECTIVE: This study clarified characteristics of interferon-associated type 1 diabetes. RESEARCH DESIGN AND METHODS: The study compared 12 patients with interferon-associated type 1 diabetes with 128 patients with type 1A diabetes with respect to clinical characteristics, and with 10 patients without diabetes despite interferon therapy and 136 normal controls with respect to HLA allele distributions. RESULTS: Patients with interferon-associated type 1 diabetes retained higher levels of fasting serum C peptide as well as GAD65 antibodies than those with type 1A diabetes until 2 to 4 years after onset. HLA-A*2402 was increased among patients with interferon-associated type 1 diabetes compared with those without diabetes, despite interferon therapy (odds ratio [OR] 4.00 [95% CI 1.09–17.26]). The haplotype of DRB1*1302-DQA1*0102-DQB1*0604 was increased in these two groups combined compared with normal controls (OR 5.64 [95% CI 2.67–11.81]). CONCLUSIONS: Interferon-associated type 1 diabetes is characterized clinically by high titers of GAD65 antibodies and preserved β-cell function, and genetically by addition of HLA-A*2402to DRB1*1302-DQA1*0102-DQB1*0604.